1. Field of the Invention
The present invention relates to a process for the preparation of rubber-containing graft copolymer particles having excellent powder properties, to thus improve the impact resistance of rigid resins such as polyvinyl chloride, polystyrene, polymethyl methacrylate and acrylonitrile-styrene copolymer, or resins known as "engineering plastics", such as polycarbonate, PET, PBT, and polyacetal.
2. Description of the Related Art
It is well known to blend a rubber-containing graft copolymer with rigid resins such as polyvinyl chloride, polystyrene, polymethyl methacrylate and acrylonitrile-styrene copolymer, or resins called "engineering plastics" such as polycarbonate, PET, PBT, and polyacetal, to thereby improve the impact resistance of these resins. The rubber-containing graft copolymer is obtained usually as powder particles, generally by coagulating a latex prepared by emulsion polymerization with an acid, a salt or the like, and then dehydrating and drying the thus-coagulated latex.
Nevertheless, these powder particles are likely to give rise to serious problems in that caused blocking occurs during the storage thereof, and a transportation line is clogged with these particles, and so on. Further, when the rubber content of the rubber-containing graft copolymer increased, to thereby improve the impact resistance of the resin to be blended with this copolymer, the above problems become more serious, and this is inconvenient.
Accordingly, with the recent automation of a measurement and an oversizing of transportation lines, an improvement of the powder properties such as blocking resistance and powder fluidity is urgently required, and thus various processes for the improvement of the powder properties of a rubber-containing graft copolymer particles have been examined. There are known, e.g., a process in which finely powdered silica, sodium carbonate, calcium carbonate, titanium dioxide, various metallic soaps or the like is added to the powder particles of a rubber-containing graft copolymer, and a process in which a lubricant is added to such powder particles, as disclosed in Japanese Unexamined Patent Publication No. 64-2644. These processes, however, are accompanied by serious problems in that the improvement of the powder properties is insufficient, or in that even if such an improvement is observed, the imparting of an impact resistance to a rubber-containing graft copolymer is lowered, and the transparency, processability and thermal stability of the powder become poor, and so on. On the other hand, Japanese Examined Patent Publication No. 58-48584 (and corresponding British Patent Publication No. 2,044,779), discloses a process in which a graft copolymer with a low rubber content, for an improvement of the powder properties, is mixed with a graft copolymer with a high rubber content, as a slurry. According to this process, the improvement of the powder properties of a graft copolymer with a high rubber content is still insufficient, although the copolymer for improving the powder properties does not adversely affect the physical properties of the graft copolymer with a high rubber content to be used. Further, the above-described Japanese Examined Patent Publication 58-48584, discloses, as Comparative Example 3, an example where an impact resisting improving agent obtained by mixing, with a rubber-containing graft copolymer, as a slurry, a rigid non-elastic double-stage polymer free from rubber component obtained by polymerizing a mixture of styrene and methyl methacrylate at the first stage and polymerizing methyl methacrylate at the second stage, is blended with a polyvinyl chloride resin. Also, an example is given wherein, in a mixture obtained by mixing the double-stage polymer with such a composition as a slurry, when the obtained modified rubber-containing graft copolymer is blended with a polyvinyl chloride resin, many fish eyes are produced, or the impact resistance of the resin is lowered, and it is stated that the effect of the rubber-containing graft copolymer of improving the properties of the resin to be blended with said copolymer is still insufficient. In addition, Japanese Examined Patent Publication No. 61-57341 (and corresponding U.S. Pat. No. 4,097,553) or Japanese Unexamined Patent Publication No. 58-1742 (and corresponding U.S. Pat. No. 4,463,131), discloses that a rigid non-elastic polymer in a state of slurry is mixed with a rubber-containing graft copolymer, but the rigid non-elastic polymers proposed in these publications are considered to be those prepared a one-stage polymerization process, from the total descriptions of these publications. As long as such a polymer is used, sometimes problems arise, similar to those described in Comparative Example 3 of the above-cited Japanese Examined Patent Publication No. 58-48584, in that, when the modified rubber-containing graft copolymer is blended with a polyvinyl chloride, fish eyes are produced and that the impact resistance of the resin is lowered.
As described above, a process has not been found whereby the powder properties can be practically sufficiently improved without producing fish eyes and without lowering the impact resistance, transparency, processability or thermal stability of the resin, when a rubber-containing graft copolymer is blended as an impact resistance improving agent with the resin to be mixed with the copolymer.